Ball bonding and stud bump bonding are bonding processes used to bond contact pads, traces, wire leads and/or electrical connectors to form electrical connections on semiconductor devices. Gold, copper, aluminum, and various alloys are used as metals for a ball bonding. In some situations, platinum has been used as a metal “bump” for ball bonding, stud bump bonding, or other types of semiconductor device bonding. However, platinum presents significant difficulties when used as a bump metal.
Various semiconductor-based devices are configured to detect physical events and/or cause physical events. Such devices are generally known as a Micro-Electro-Mechanical Systems (MEMS) device. For example, a MEMS gyroscope may be used to determine angular rotation and a MEMS accelerometer may be used to sense linear acceleration. The MEMS gyroscope and accelerometer measure rotation and acceleration, respectively, by measuring movement and/or forces induced in one or more proof masses mechanically coupled to and suspended from a substrate using one or more flexures. As another example, a MEMS motor may be used to induce or sense movement in a rotor.
A number of recesses etched into the substrate of the MEMS device allow selective portions of the silicon structure to move back and forth freely within an interior portion of the MEMS device. A pattern of electrical connectors, also known as metal traces, are formed on the MEMS device substrate to deliver various electrical voltages and signal outputs to and/or from the MEMS device. The MEMS device, after fabrication, may be enclosed in a protective enclosure having wire leads or connectors that provide connectivity between the outside surface of the enclosure and the metal traces of the MEMS device. Ball bonding is one method that may be used to electrically couple the wire leads or contact pads of the enclosure and the metal traces of the MEMS device.
Such MEMS devices are very sensitive to inducted stresses and/or changes in orientation of the MEMS device components. Very small changes in stress and/or orientation of the working components of the MEMs device may significantly change the signal output of the MEMS device. Accordingly, prior to use in the field, the MEMS device is calibrated. Typically, calibration of the MEMS device is performed at the factory or during a field calibration process. For example, output of a stationary MEMS gyroscope or accelerometer should be null (zero). Accordingly, during the MEMS device calibration process, the output of the stationary MEMS gyroscope and accelerometer is referenced to a null value and/or is electrically compensated to a null output.
As noted above, gold is a commonly used metal for a bump, such as a ball bond, a stud bump flip chip bond, or other semiconductor device bonds. Gold is a very ductile metal that can be plastically deformed. Further, the gold may be plastically deformed multiple times during a multi-stage bonding process. Accordingly, during a thermo compression bonding process or a thermo-ultrasonic bonding process, the ductile characteristics allow deformation of the gold bump while still providing good bonding characteristics between the gold and the wire leads or contact pads. That is, the gold easily deforms under pressure to provide a relatively large contact area with the wire leads or contact pads, and also easily electrically bonds to the wire leads or contact pads in response to the applied pressure, heat, and/or ultrasonic energy.
However, the ductility of a gold bump bond (and bump bonds made of copper, aluminum, and other alloys) has significant drawbacks when used for bonds of a MEMS device. Temperature fluctuations of the MEMS device causes a heat-induced deformation of the bump bond. Because of the ductility of the gold bump bond, such temperature-induced deformations are, at least to some extent, nonelastic. That is, after a number of temperature cycles, the deformed gold bump bond does not return to its original pre-deformation stress and/or form. Such nonelastic deformations in the gold bump bond is referred to as creep.
In the MEMS device, such nonelastic deformations in the gold bump bond may result in the MEMS device becoming uncalibrated. That is, the creep of the gold bump bond induced by temperature cycling may change the stresses at the working element of the MEMS device. In some situations, the creep of the gold bump bond induced by temperature cycling may change the position and/or orientation of the working element of the MEMS device. Accordingly, the MEMS device will become uncalibrated.
Annealed platinum metal initially exhibits very desirable ductile characteristics for a bump bonding process. However, once an annealed platinum bump has been subjected to forces that are sufficient to realign the crystal orientation of the platinum atoms, the platinum changes from an annealed material to a non-ductile material that is relatively brittle. A work hardened (also referred to as strain hardened or cold worked) platinum bump bond does not exhibit creep after temperature cycling.
However, once a platinum bump is work hardened, the platinum does not readily deform during a subsequent bonding stage. That is, the work hardened platinum does not easily deform to provide the desired relatively large contact area with the wire leads or contact pads. Further, the work hardened platinum bump does not easily electrically bond to the wire leads or contacts in response to the applied pressure, heat, and/or ultrasonic energy. Bonding of a work hardened platinum bump requires a relatively large amount of pressure, temperature, and/or ultra-sonic energy, which may damage a relatively fragile MEMS device.
FIGS. 1-3 illustrate selected stages of a prior-art gold stud bump bonding process that uses platinum as the ball, or stud bump, metal. FIG. 1 illustrates a platinum wire 102 threaded through the capillary of a threaded capillary bond tool 104. An annealed platinum ball 106 is formed on the end of the platinum wire 102 by melting a portion of the platinum wire 102 extending out from the chamfered end 108 of the threaded capillary bond tool 104. Heat, which melts the platinum wire 102 to form the annealed platinum ball 106, may be generated by an electrical arc or by a flame.
FIG. 2 illustrates bonding of the annealed platinum ball 106 to a contact pad 202. The chamfered end 108 of the threaded capillary bond tool 104 is shaped so as to deform the annealed platinum ball 106 into a desired shape as force is exerted by the threaded capillary bond tool 104 onto the annealed platinum ball 106. Since the contact pad 202 is stationary during the bonding process, the applied force deforms the annealed platinum ball 106 into a bump. Additionally, heat and/or ultrasonic energy may be applied as the annealed platinum ball 106 is pressed against the stationary contact pad 202, thereby forming an electrical contact 204 at the juncture of the annealed platinum ball 106 and the contact pad 202.
The threaded capillary bond tool 104 is then withdrawn and the platinum wire 102 near the top 302 of the platinum ball 106 is cut or sheared off, leaving the deformed platinum ball 106 that has been bonded to the contact pad 202, as illustrated in FIG. 3. To complete the electrical connection process, such as with a stud bump flip chip bond, a lead or another contact pad must be bonded to the top 302 of the deformed platinum ball 106. However, since the applied force deforms the platinum ball 106, the deformed platinum ball 106 has become work hardened. Thus, the work hardened platinum ball is now relatively difficult to further deform and bond with a wire lead or contact pad during a subsequent bonding process.
Accordingly, the subsequent bonding process that bonds the work hardened top of the deformed platinum ball 106 with a wire lead or another contact pad requires a relatively large amount of force, heat, and/or ultrasonic energy (compared to the amount of force, heat, and/or ultrasonic energy used to bond the annealed platinum ball 106 to the contact pad 202). Alternatively, a relatively large amount of heat may be applied to the work hardened deformed platinum ball 106 to re-anneal the platinum. However, the relatively large amount of force, heat, and/or ultrasonic energy required to subsequent bonding of the top 302 of the deformed platinum ball 106 may damage a relatively fragile MEMS device.